Abstract

The experimental design in a field plot trial (soil pH 5.8; OM
17%) conducted in the rainy season (September to October 2012) involved 24 treatments
arranged in a 6*4 factorial
arrangement with 3 replications. The first factor was level of biochar (0, 1, 2,
3, 4 and 5 kg/m2); the second factor was the type of vegetable (Water
spinach, Chinese cabbage, Celery cabbage and Mustard green). Fertilization
was with biodigester effluent (10kg N/ha applied to all treatments. The area of each
plot was 1.6m2
(2.0m length x 0.8m width) with spacing between each plot of 0.5m. The
experiment lasted 35 days. The biochar (pH 9.3; OM 29.4% in DM) was from a paddy
rice drier (combustion temperature with rice husks as feedstock was about 500°C).

Increasing the application of biochar from 0 to 5 kg/m2
led to linear increases in biomass DM yield of 39, 100, 300 and 350 % for
Water spinach, Chinese cabbage, Celery cabbage and Mustard green, respectively.
Soil quality was improved after the 35 day trial (pH 6.82-7.13; OM 22.6 -
25.7%). The chemical composition of the biomass DM showed average increases in
crude protein from 13.7 to 18.1% for leaves and from 7.23 to 9.16 for stems.
By contrast, crude fiber in leaves decreased from 14.5 to 9.27% in DM while in
stems it fell from 15.6 to 10.7%.

Introduction

In Cambodia,
diesel fuel for running the generator in rice mills is increasing in price and
prices are not stable, thus rice mill owners try to find alternative ways to
reduce the cost of fuel. Following the introduction to Cambodia of an
Indian-made gasifier in 2004 by CelAgrid (Phalla and Preston 2005), several
local companies have begun to construct gasifiers using rice husks as the
feedstock. Gasification offers a more sustainable pathway as a means to extract
energy from renewable biomass. Gasification is the way to convert solid fibrous
biomass by pyrolysis into producer gas which can be used as fuel for internal
combustion engines and gas turbines, as well as a source of heat. Additional
benefits are that the system is “carbon-neutral” (does not add to global
warming), produces negligible amounts of sulphur compounds (the cause of “acid”
rain), reduces waste disposal and has fewer negative environmental impacts. The
residue from biomass after processing in a gasifier is known as BIOCHAR.

Biochar is a
fine-grained porous substance that resembles charcoal produced by natural
burning.However, biochar is produced by the combustion of
biomass under oxygen limited conditions at high temperatures (from 600 to
1000 °C) in a gasifier. As most of the mineral matter in biomass is composed of
salts of K, Na and Ca, it has a strong alkaline reaction giving rise to a pH of
between 8 and 10 (Rodriguez et al 2009). Thus application of biochar as a soil
amender is especially appropriate in acid soils with a low content of organic
matter. Biochar is unlikely to have a major role as a fertilizer but, because of
its structure, it can be expected to increase water-holding capacity, and be a
good habitat for microbes and plant nutrients. A study by Rodriguez et al (2009)
using 50g biochar/kg soil applied to fertile soil or sub-soil with and without
effluent of 100 kg N/ha, showed that biochar with effluent markedly increased
the biomass growth of maize especially on the poor sub-soil.
Soil pH was increased from 4-4.5 to 6.0-6.5 due
to addition of biochar.

In Cambodia, most farmers after
rice is harvested burn the rice straw in order to facilitate subsequent plowing,
but this has negative effects on soil fertility as organic matter is lost and
valuable soil microbes are destroyed. Application of biochar which increases
water holding capacity and is a good habitat for plant nutrients and microbes
can thus bring about improvement in soil fertility and yield of rice. This was
recently demonstrated in Cambodia in recent studies by Boun
Suy Tan (2010) (http://biocharinnovation.wordpress.com/workshop-cambodia)
and Sokchea et al (2013). In the former case, the rice yields
were more than doubled (3.76 tonnes/ha) by application of 40 tonnes/ha of
biochar compared with the control (without biochar) which yielded only 1.25
tonnes/ha. In the second study, application of 30 tonnes/ha biochar to a rice
crop increased grain yield by 30% and straw yield by 40%.

In Vietnam, biochar has been
applied successfully to increase the yield of mustard green vegetable
(Pham Van Luu et al 2013). However, there appear to be no reports in Cambodia on
the use of the biochar for growing vegetables.

The present study examined the
effect of biochar on the production and nutritive value of a range of vegetables commonly grown by
local farmers in Cambodia.

Materials and Methods

Location

The experiment was carried out at
the Center for Livestock and Agriculture Development (CelAgrid) located in Prah
Theat village, Sangkat Rolous, Khan Dangkor, approximately 25 km from Phnom Penh
city. The experiment was conducted in the rainy season from September to October
2012.

Experimental design

The experiment design was a
Randomized Complete Block (RCBD) with 24 treatments in a 6*4 factorial
arrangement with 3 replications. The first factor was level of biochar (0, 1, 2,
3, 4 and 5 kg/m2); the second factor was the type of vegetable (Water
spinach - WS, Chinese cabbage – ChC, Celery cabbage – CeC, and Mustard green -
MG).

Land preparation

The land was plowed 2-3 times and
sun-dried for a week before making the beds; the area of each bed was 1.6m2
(2.0m length x 0.8m width) and spacing between each bed was 0.5m.

Planting vegetables

Chinese cabbage, celery cabbage and mustard green were germinated in a nursery
without applying any fertilizer and the best plants selected for transplanting
at 14 days with 20 cm distance from plant to plant. The dry-land water spinach
species was chosen for this species. The seeds were kept in water at ambient
temperature for 12 hours before planting and seeding density was 62.5g/m2.

Irrigation and biochar application

The biochar was the residue from
combusting rice husks in a paddy rice dryer in which the furnace temperature was
from 500 to 6000C. This temperature is similar to that in a
conventional down-draft gasifier and it has been shown that there were no
differences in the yield response of rice to biochar from the paddy rice dryer
and a conventional gasifier (Sokchea et al 2013). The biochar was
incorporated in the upper 10 cm of the soil in each of the beds (thus 1 kg/m2
is equivalent to 10 tonnes/ha).

The biodigester effluent was from
a fixed dome brick and concrete model which had a capacity of 15m3.
It was charged with manure from pigs fed a commercial concentrate feed. All
treatment plots received the same level of biodigester effluent at a level of 10
kg N/ha. This was applied in amounts equivalent (as a proportion of the total application)
to 20% at 14 days (time of transplanting), 40% at 21 days and 40% after 28 days.
The effluent was diluted with water in proportions of 50:50 (fresh basis) before
application. The plots were irrigated uniformly based on weather conditions,
usually around 2 times a day.

Measurements

The plant height and numbers of
leaves were measured at 14, 21 and 28 days after planting. At the end of the
experiment (35 days), representative plants were harvested including the roots
in order to measure total biomass yield of the vegetables.

Chemical analysis

Soil samples were analyzed before
and after completing the experiment). Soil and biochar samples were analyzed for
pH, organic content (OM) and N by methods from "Soil chemical analysis":
http://www.icarda.org/Publications/Lab_Manual/PDF/part5.pdf). The
biodigester effluent was analyzed for DM and N at each application. The
vegetable biomass was analyzed for moisture, organic matter, N and crude
fibre following the methods in AOAC (1990), except for moisture content which
was determined using the method of Undersander et al. (1993); ash and N were
done according to the methods of AOAC (1990).

Statistical analysis

The data were recorded in MS
Excel and
analyzed by the General Linear Model option in the Analysis of Variance (ANOVA)
program of the Minitab software (2000). Sources of variation
were levels of biochar, type of vegetable, interaction between level of biochar
* type of vegetable and error.

Results and discussion

Chemical composition of biochar and effluent

The values for OM content and pH of the biochar from the
rice dryer can be compared with values of 10.3% (for OM) and 10.9 (for pH)
reported by Sokchea et al (2013). More precise methods to characterise
biochar (eg: the BETS measurement of surface area) are being used but were
not available for the present study.

Table 1:
Chemical composition of biodigester effluent and biochar

Dry matter
%

OM
% in DM

N
mg/liter

pH

Biodigester
effluent

0.75

ND

670

ND

Biochar

63.0

29.4

ND

9.30

ND: not
determined

Effects of biochar on the soil

The soil
used in the experiment showed improvements as reflected in increased content of organic matter, nitrogen and
pH as a result of the addition of biochar. The effects were most notable between
zero and 1% biochar, with little change noted for higher applications of biochar
(Table 2). Similar responses have been reported in soils that were amended with
biochar (eg: biochar from a downdraft gasifier [Rodriguez et al 2009], from an
updraft stove [Southavong et al 2012] and a paddy rice dryer [Sokchea et al
2013]).

Table 2:
Chemical composition of soil before and at the end of experiment
(Organic matter and Nitrogen as % of DM)

Level of biochar, kg/m2

0

1

2

3

4

5

Soil before

Organic matter

---------------- 17.3
-------------------

Nitrogen

--------------- 0.30
-------------------

pH

--------------- 5.80
-------------------

Soil at the end

Organic matter

18.5

22.6

25.2

24.1

23.9

26.3

Nitrogen

0.27

0.39

0.33

0.37

0.37

0.38

pH

6.18

6.82

7.09

7.13

7.06

7.29

Effects of biochar on the chemical composition of the plants

There were linear changes in the composition of the leaves and stems of the
vegetable (Table 3; Figures 1 and 2). The DM content of leaves and stems was not
affected by the level of biochar that was applied; however, in both leaves and
stems the crude protein increased (on average by some 30%) and the crude fiber
decreased (by some 30%) as the application of biochar was increased from zero to
5 kg/m2.Responses were similar for the different
vegetables (P for the biochar*species interaction was 0.66).

Figure 1:
Relationship between level of biochar and crude protein content of
leaves and stems of four vegetables (mean values of Celery cabbage,
Chinese cabbage, Mustard green and Water spinach)

Figure 2:
Relationship between level of biochar and crude fiber content of
leaves and stems of four vegetables (mean values of Celery cabbage,
Chinese cabbage, Mustard green and Water spinach)

Growth in height and increase in leaves

Both growth in height and in the
numbers of leaves recorded for the different vegetables showed
linear increases to level of added biochar (Tables 4 and 5). These effects
of treatments are discussed in detail in the subsequent section on biomass
yields.

Table 4: Effect of
different level of biochar on the height (cm) of Celery cabbage (CeC),
Chinese cabbage (ChC), Mustard green (MG) and Water spinach (WS)

Level of biochar, kg/m2

Type of vegetable

0

1

2

3

4

5

SEM

Prob

Cec

ChC

MG

WS

SEM

Prob

14 days

11.4

12.3

14.5

14.4

16.1

16.9

0.58

<0.001

19.0

12.9

16.2

8.97

0.47

<0.001

21 days

14.0

15.0

17.6

17.5

19.8

20.9

0.70

<0.001

21.4

14.8

18.6

15.1

0.57

<0.001

28 days

16.9

17.6

20.8

20.4

24.1

25.1

0.81

<0.001

24.8

17.0

22.1

19.2

0.66

<0.001

Growth, cm/day

0.48

0.50

0.59

0.58

0.69

0.72

0.02

<0.001

0.71

0.49

0.63

0.55

0.02

<0.001

Table 5: Effect of
different level of biochar on number of leaves of Celery cabbage (CeC),
Chinese cabbage (ChC), Mustard green (MG) and Water spinach (WS)

Level of biochar, kg/m2

Type of vegetable

0

1

2

3

4

5

SEM

Prob

Cec

ChC

MG

WS

SEM

Prob

14 days

6.00

6.22

6.64

6.72

7.19

7.67

0.18

<0.001

5.94

10.7

5.98

4.35

0.15

<0.001

21 days

7.58

7.81

8.56

8.58

9.08

10.3

0.22

<0.001

7.89

12.3

7.98

6.44

0.17

<0.001

28 days

9.31

9.64

10.8

10.8

11.4

12.9

0.32

<0.001

10.3

14.1

10.2

8.57

0.26

<0.001

Biomass yield

For each type of vegetable, application of increasing quantities of biochar led
to positive linear or curvilinear increases in biomass yield of leaves, stems
and roots (Table 6; Figures 1-4). Yield increases for biochar application of 5
kg/m2 (50 tonnes/ha) were of the order of 300%, 100%, 350% and 39%
for Celery cabbage, Chinese cabbage, Mustard green and Water spinach,
respectively. Responses were much less for water spinach than for the three
types of cabbages.

Figure 6.
Relationship between level of biochar and green biomass DM yield of
water spinach

Biomass yield: proportions of
leaf, stem and root

The vegetables differed markedly in the proportions of leaves and stems, the
proportion of the latter being much higher in Water spinach (Table 6). The
vegetables also responded differently to soil amendment with biochar. For the
Chinese cabbage and Mustard green (Figures 8, 9 and 11), the proportions of
leaf, stem and root showed little change with increasing yield. However, in the
case of Celery cabbage and Water spinach, the response was quite different as
the increase in biomass yield was reflected in a decrease in the yield of leaf
and increase in stems (Figures 7, 10 and 11).

Table 6: Effect of
different level of biochar on the proportion of Celery cabbage (CeC),
Chinese cabbage (ChC), Mustard cabbage (MG) and Water spinach (WS)

Level of biochar, kg/m2

Type of v egetable

0

1

2

3

4

5

SEM

Prob

CeC

ChC

MG

WS

SEM

Prob

Proportion of biomass

Leaves

44.1

44.2

41.1

43.8

42.6

43.9

1.39

0.583

42.0

53.7

51.6

25.9

1.13

<0.001

Stem

38.6

40.0

42.8

40.5

41.9

41.8

1.43

0.361

41.7

31.2

34.4

56.4

1.17

<0.001

Root

17.3

15.8

16.1

15.7

15.5

14.3

0.83

0.268

16.3

15.1

14

17.7

0.67

<0.001

Proportion of plant

Leaves

53.1

52.4

48.8

51.5

50.4

51.2

1.57

0.478

50.2

63.2

60.1

31.5

1.28

<0.001

Stem

46.9

47.6

51.2

48.5

49.6

48.8

1.56

0.476

49.8

36.8

39.9

68.5

1.27

<0.001

Figure 7: Relationship between level of biochar and DM
biomass yield of Celery cabbage as root, stem and leaf

Figure 8: Relationship between level of biochar and DM
biomass yield of Chinese cabbage as root, stem and leaf

Figure 9: Relationship between level of biochar and DM
biomass yield of mustard green as root, stem and leaf

Figure 10: Relationship between level of biochar and DM
biomass yield of Water spinach as root, stem and leaf

Figure 11. Effect of level of biochar on change in
proportion of leaf biomass as proportion of leaf + stem

Apart from the greatly enhanced yield of the
vegetables, the improvement in nutritive value (more protein; less fiber) as a
result of soil amendment with biochar is especially promising. This appears to
be the first report from a replicated field experiment showing these changes. A
long term field demonstration in Canada (latitude 45°N),
in which biochar was applied at 3.9 tonnes/ha to mixed grass-clover forage plots
(Husk and Major 2011), demonstrated increased yields of forage (4.1%) in
the third year and associated increases in nutritive value (Crude protein
increased by 10%, NDF decreased by 5.9%; predicted improvement in milk
production from 20 to 44%). However, the trial plots were not replicated and the
forage composition of the plots also changed, with the biochar plot
showing clover increasing from 35 to 51% of the sward and ryegrass decreasing
from 60 to 40%. These changes in botanical composition would explain part of the
changes in nutritive value. Nevertheless, these findings lend support to the
results we report from the present study, and urge the necessity for more
detailed long-term research on the effects on plant composition in
soils amended with biochar.

It is difficult to develop an explanation for these marked effects of biochar on the
nutritive value of the vegetables, but if confirmed in other studies this would
represent a major virtue to be added to the list of attributes apparently
possessed by biochar as a component of farming systems, at least for biochar
derived from rice husks.

Conclusions

Increasing the application of
biochar to a fertile soil (pH 5.8; OM 17%) from 0 to 5 kg/m2
during a 35 day trial led to linear increases in biomass DM yield of
39, 100, 300 and 350 % for Water spinach, Chinese cabbage, Celery cabbage
and Mustard green, respectively,

Soil quality was inproved
after the 35 day trial (pH 6.82-7.13; OM 22.6 to 25.7%).

The chemical composition of
the biomass DM showed average increases in crude protein from 13.7 to 18.1%
for leaves and from 7.23 to 9.16 for stems. By contrast, crude fiber
in leaves decreased from 14.5 to 9.27% in DM while in stems it fell from
15.6 to 10.7%.

Acknowledgements

The authors would like to express
their gratitude to the MEKARN project financed by the SIDA-SAREC Agency and to
the Center for Livestock and Agriculture Development (CelAgrid), for providing
resources for conducting this experiment.